The biggest benefit Kyu-b can bring to a team is through its mixed set. Obviously a physical attacker with a strong ice STAB is going to be excellent in a tier where all the dominant physical walls are weak to ice. Notable dragon resists like Ferrothorn and Skarmory are also weak to its coverage.The mixed set has far fewer checks though. Some require prediction to be forced out, such as steel arceus, but many others, like Groudon, Ferrothorn, and Dialga, really can't switch into the vast majority of its moves and come out healthy enough to wall anything else, making it a decent pokemon to put pressure on the opposing team.
It can trouble almost all dedicated physical walls, and if using the set it should be using, (Outrage/hp fire if sunny day, Focus Blast if not/Ice beam/Fusion Bolt), it can take advantage of its actually not too terrible defensive typing against rain based teams to find attacking oppurtunites, and its coverage and power against common rain cores is pretty hard to ignore. It can dent ferrothorn pretty heavily, and can actually beat it in the sun if running hp fire, and can definitley put a huge dent in it, allowing the myriad of sweepers checked by it a free pass. It also puts a ton of pressure on Ferro/tenta/kyogre cores, the somewhat standard rain team.
Against sun its biggest niche is obviously being a physical threat that can ignore groudon as a check or a wall until it locks itself into outrage. Another notable niche is not having to lock itself into a move to pose a major threat to ho-oh.
In my opinion the need to run LO to be effective, as well as being SR weak, really cut down on the value of the resistances that make the a fore mentioned positives of kyu-b possible to execute in a regular game situation. It's pretty good as a wall breaker and certainly has some interesting niches, but overall I would usually pick something like mix rayquaza over it, because though it can't do certain things kyu-b can, with just a little support it can keep most of kyu-b's niches whilst offering much, much more general utility.
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